1. Field of the Invention
This invention relates to a process for producing conference connections in a PCM time multiplex switching system, and apparatus for carrying out the process, according to which the total sum code words are formed from the PCM words supplied by the conference parties, with the formation of intermediate sum code words which are stored in a conference memory, and, in each case, with the occurrence of a new PCM word supplied by a conference party, are read and added to this PCM word and then stored again as a new intermediate sum code word. Correspondingly delayed PCM words supplied in a prior pulse frame are subtracted from the new intermediate sum code word during the time slots assigned to the conference parties, and the final sum code words are respectively sent to the conference party whose sum portion is not contained in such word.
2. Description of the Prior Art
It is generally known to carry out a process of the type set forth above. For example, in U.S. Pat. No. 3,551,600 an adder and a first conference memory are provided for forming the intermediate sum code words or, respectively, the total sum code words. In this arrangement, the PCM words supplied by the individual conference parties are fed to one input of the adder and the intermediate sum code words which are read from the conference memory simultaneously with the occurrence of the PCM words and which are formed from previous PCM words are fed to the other input of the adder so that with each addition new intermediate sums are formed which contain the conference portion identifying a further conference party. These intermediate sum code words are then stored into the conference memory until the formation of a further intermediate sum or, respectively, finally, of the total sum code word. After the conclusion of a summing cycle of this kind, the total sum code word is transferred to a second memory which forms a component of a conference circuit of this kind, from which it is repeatedly read, in each case, during the time slots assigned to the individual conference parties, and is also fed to one input of a subtractor. The other input of the subtractor receives the PCM words of the individual conference parties, delayed by one pulse frame with the aid of a delay circuit. In this manner, during the time slots assigned to the conference parties, partial sum code words are formed, which do not contain the conference portion of another conference party.
The summing of the PCM words of the parties into a total sum code word, as well as the transfer of the word into the second memory of the conference circuit must occur within the time period of one pulse frame. Particularly in those cases in which a conference involves conference parties whose time slots are spaced nearly one pulse frame apart from one another, i.e. to whom, for example, the first and the last channel of a pulse frame are assigned, it is not readily guaranteed that PCM words or, respectively, intermediate sum code words and total sum code words simultaneously required for processing are also simultaneously available, and that writing and reading operations do not overlap on one and the same memory cell.